1. Field of the Invention
This invention relates to magnetic resonance imaging systems.
2. Description of the Related Art
In order to perform magnetic resonant imaging, it is necessary to provide a magnetic field gradient in spatial directions to encode locations of the sources of a magnetic resonance (MR) response signal. This encoded MR response signal is used to create the MR images. In conventional MR imaging systems there is a set of gradient coils through which current flows to produce a magnetic field which either adds or subtracts to a static magnetic field provided by a main magnet.
In order to create a magnetic field gradient, required for MR imaging, current must be provided over a surface with a specified current density. Since current requires a closed loop, there is a return loop which typically provides a magnetic field which is not the same as the desired magnetic field gradient. Typically the return loop distorts and weakens the desired gradient magnetic field.
The magnetic field gradient is intended to be created within an imaging volume inside the MR imaging apparatus. Stray magnetic fields which are produced outside of the imaging volume by the currents in the gradient coil can induce stray currents, known as eddy currents, in electrically conducting support structures in the magnet. The eddy currents produce additional and undesirable gradient fields within the imaging region. It is, therefore, desirable to contain the stray magnetic fields in order to restrict their ability to induce eddy currents.
One method of containing the stray magnetic fields is to provide a second layer of coils outside, and surrounding, the first layer of coils used to produce the magnetic field gradients. This second layer of coils produces a magnetic field of opposite polarity compared to that of the first layer, and neutralizes most of the magnetic field produced by the first layer of coils outside of the imaging volume. This requires driving current through an additional layer of coils. The power required to drive a current through coils is related to the strength of the desired gradient magnetic fields necessary to carry out a prescribed imaging process. Since large changes in current are desired to be applied over very short time periods, the power required can become very large. This, combined with the fact that there is a second layer of coils to drive, results in a large power requirement of the MR imaging apparatus.
Since the speed of the MR imaging, and the quality of the imaging data, is related to how quickly the current can be changed within the gradient coils, a two layer, or shielded, gradient coil design is inherently slower than the single layer design using the same coils.
The need for a return current makes the gradient coils longer, thereby causing some patients to become uncomfortable and claustrophobic when placed inside the imaging system.
Currently there is a need for an MR imaging system which contains stray magnetic gradient fields, also is faster for a given gradient amplifier power, and is not as confining as conventional magnetic resonance imaging systems.